High voltage components and devices, such as insulators, surge arresters and cable terminations may be provided with one or more sheds to increase the tracking length. Tracking is the well-known phenomenon of leakage currents flowing over the outer surface of the component. Lengthening the leakage path increases its resistance and thereby reduces the current and any surface deterioration caused by the leakage current. In this context, the term high voltage is understood to include voltages of more than 400V, in particular more than 1000V, and especially more than 5000V.
Traditionally, several individual ring-shaped sheds are arranged on the outer surface of a high-voltage component. The sheds may be heat-shrinkable, as disclosed in International Patent application WO 94/29886 (Raychem). The sheds may be combined into a single component, as disclosed in U.S. Pat. No. 5,389,742 (Raychem). In all these arrangements, the sheds constitute an array of approximately ring-shaped elements. In contrast, U.S. Pat. No. 4,833,278 (Hydro-Quebec) discloses an essentially helical shed made up of several joined shed segments. The above-mentioned U.S. Pat. No. 5,973,272 suggests to wind a single T-shaped shed element around a tube so as to provide an uninterrupted helical shed. The T-shaped element can be continuously extruded by an extruder the head of which is arranged at approximately a right-angle relative to the tube. The T-shaped shed element can be wound around the tube as it is being extruded, allowing a single component to be provided with a helical shed in one single process step. Although the process of U.S. Pat. No. 5,973,272 is very advantageous, the use of a T-shaped structure necessarily limits the bend radius of the helical shed. For this reason, this known process is not suitable for components having a relatively small diameter. In addition, the mutual sealing of adjacent windings of the T-shaped shed element cannot be guaranteed. As a result, dirt may accumulate in any gap between the windings and may decrease the surface resistance of the component, thereby causing an increased amount of tracking or water may penetrate the seal and cause electrical failure in the substrate. Another process, described in WO-A-99/10896, similarly uses transverse extrusion of the shed, resulting in bond lines between adjacent turns of the shed.